Microelectromechanical systems (MEMS) technology is increasingly being implemented for providing many products, such as inertial sensors, accelerometers for measuring linear acceleration, gyroscopes for measuring angular velocity, optical devices, pressure sensors, switches, and so forth. A MEMS device typically includes a moveable element, such as a proof mass, diaphragm, mirror, and the like that is flexible or movable, and is attached to the rest of the device. Relative motion between this movable element and the rest of the device is driven by actuators and/or sensed by sensors in various ways, depending on device design.
Semiconductor processing, used for fabricating MEMS devices, generally comprises multiple photolithographic, etching, depositing, and doping operations to form an array of individual MEMS devices on the surface of a semiconductor substrate, such as a wafer. Semiconductor processing for MEMS devices, typically entails one of bulk- and surface-micromachining. In bulk-micromachining, MEMS features are created by selectively removing silicon to form the desired structures. In surface micromachining, an additive process is performed using polysilicon layers and/or metal layers on top of sacrificial oxides and then removing the sacrificial layers to create the MEMS devices. Each MEMS device is separated from the others by a narrow inactive, i.e., unused, region on the device wafer referred to as a die “street”. Following micromachining and wafer level testing, individual MEMS devices are “singulated.” Singulation is typically accomplished by sawing or cutting along scribe lines in the die streets to produce singulated semiconductor dies.
Most MEMS devices require terminal elements, in the form of electrical inputs and outputs, to perform their design functions. Traditionally, MEMS devices require custom cavity based packaging to both provide access to the input/output elements and to protect the MEMS features, which are generally very fragile and sensitive to dust, particles, and moisture. The packaging for a MEMS device can entail a protective cover over the sensitive MEMS features that will allow the part to be handled by standard assembly and packaging means. One conventional method has been to have pre-fabricated individual covers that are picked and placed over the sensitive MEMS features by automated means prior to dicing the MEMS device wafers. Another packaging method is to provide for protective covers by etching cavities in a silicon cap wafer and affixing it to the MEMS device wafer by various means, such as solder, glass frits, adhesives, and so forth.
A challenge faced in performing any of the protective capping techniques has been to allow for ready access to the terminal elements. In MEMS wafer processing, a release step may be performed that exposes the terminal elements in order to make them accessible. The release step may be performed by etching or sawing a portion of the cover or cap wafer that is not protecting the MEMS device but is obscuring access to the terminal elements. Unfortunately, such release methods can generate debris that can damage the terminal elements.